Data networks facilitate communication between several users by networking the individual users, which is to say by connecting them to each other. Communication here refers to the transmission of data between the users. The data being transmitted is sent in the form of data telegrams, which is to say said data is assembled into packets and sent to the relevant recipient in that form over the data network. The term “data packets” is accordingly also employed. In this document the terms “transmitting data” and “transmission of data” are used fully synonymously with the above mentioned transmission of data telegrams or of data packets. Networking itself is achieved, for example in the case of switchable high-performance data networks, in particular Ethernet, by switching in each case at least one switching unit between two users which is connected to both users. Each switching unit can be connected to more than two users. Users are each connected to at least one switching unit, but not directly to another user. Users are, for example, computers, stored-program controls (SPS), or other components which exchange electronic data with other components and, in particular, process it. Switching units are, for example, what are termed switches, hubs, bridges, routers, and the like. In contrast to bus systems, where users can each reach any other data network user directly over the data bus, the switchable data networks exclusively contain point-to-point connections, which is to say a user can only reach all other users in the switchable data network through forwarding by means of one or more switching units of the data being transmitted. In distributed automation systems, for example in the area of drive engineering, specific data must arrive at the intended users' location and be processed by the recipients at specific times. This is referred to as realtime-critical data or data traffic, because failure of the data to arrive at its destination at the due time will have undesired consequences for the user.
DE 100 58 524 A1 describes a system and a method facilitating both realtime-critical and non-realtime-critical communication in a switchable data network consisting of users and switching units, for example of a distributed automation system, by means of a cyclical operating mode. In what is termed a transmission cycle, for all users and switching units of the switchable data network there is in each case at least one sector for transmitting realtime-critical and at least one sector for transmitting non-realtime-critical data, as a result of which the realtime-critical communication is separated from the non-realtime-critical communication. As all users and switching units are always synchronized with a common time base, the respective sectors for transmitting data occur in each case at the same time for all users and switching units, which is to say the realtime-critical communication takes place simultaneously independently of the non-realtime-critical communication and so is not influenced by it. The realtime-critical communication is planned in advance. The injection of the data telegrams at the original sender's side and the forwarding of said telegrams by means of the switching units involved takes place on a time-controlled basis. Through buffering in the respective switching units it is accomplished that any internet-enabled non-realtime-critical communication spontaneously occurring at any time is shifted into that transmission sector of a transmission cycle which is provided for the non-realtime-critical communication and is also only transmitted there.